chore: fix default value for undefined `cfg.enable`

[mdqst]

Fixed an issue where cfg.enable would not default to false if undefined, ensuring the config block is skipped when missing or set to false. Also used lib.mkIf for proper conditional application.

[zimbatm]

@mdqst do you have a way to reproduce this?

That PR doesn't make sense to me. In my mental model, cfg.enable always exists and defaults to false since it's defined by the module system.

That being said, this is the second submission. https://github.com/numtide/treefmt-nix/pull/333 also pointed to the same issue

[zimbatm]

It feels like AI